I am 17 weeks pregnant and feel the same, though I come in to the office every day .
First of all, each pregnancy is its own animal. You can't take for granted what happened last time. It could be that your first one took so much out of you and you have not replenished. I don't know your age, but that might have something to do with it. Are you taking your vitamins? You know that your little one is taking all that he/she needs, leaving you to find more. If anybody's gonna miss out on nutrients, you're the loser.
Sometimes sleep just comes over me like a fog, and I don't see it coming. I have to find an empty office to nap. I don't always have time to do that, but I'm good to nobody if I don't. Period. What helps me is to go ahead and get up when I wake up at ungodly hours. I get something done, even if it's just taking my bath. Then, I might get some more sleep before it's time to leave for work. If not, I just go in early. This gives me time for my nap later. When I get home, I go to bed, unless I'm not ready. Bottom line--I work around my sleep, because once sleep comes, I cannot ignore it.
I won't even get into my eating schedule, but I will say that sometimes eating some juicy fruit (maybe citrus) will wake me up a bit. I tend to have a very healthy diet (never been a junker), so I snack all day on fruits and veggies. Potatoes tend to give me something solid. I have a gluten sensitivity/intolerance--which drains my energy, making me feel lethargic and depressed--but oatmeal and bagels and sandwiches feel me up and I can sleep through the night and actually get things done without feeling weak.
I think that right now it might just be a matter of getting to know yourself during this particular pregnancy. So far, you seem to be in denial. Superwoman does need her rest.
Congratulations on this pregnancy, and here's to a beautiful and healthy baby on the other side!